Common Public Radio Interface (CPRI) is a standard established by several communication equipment manufacturers for an interface between an Radio Equipment Controller (REC) and an Radio Equipment, radio equipment (RE) among radio base station equipments, which may effectively make a product classification for a radio base station and independently develop REC and RE technologies. According to the definition of the CPRI standard, there is a clear division between the functions of the REC and the functions of the RE, where the REC relates to network interface transmission, radio base station control and management, and digital baseband processing, and the RE relates to analog radio frequency functions, such as filtering, modulation, frequency conversion, and power amplification.
The CPRI interface equipments REC and RE may operate in an Frequency Division Duplex (FDD) mode or a Time Division Duplex, time division duplex (TDD) mode. In a TDD system, the conventional method is to allocate one optical fiber respectively for uplink transmission and downlink transmission to complete transmission of uplink and downlink baseband digital signals between the radio equipment controller REC and the RE. Because of the TDD time division duplex transmission mode, in the downlink transmission direction from the REC to the RE, padding bits are inserted in CPRI frames when there is no downlink data transmission; similarly, in the uplink transmission direction from the RE to the REC, padding bits are inserted in CPRI frames when there is no uplink data transmission, as shown in FIG. 1.
In this conventional transmission method, because the inserted padding bits carry no baseband digital signals, the transmission bandwidth utilization of the CPRI interface is only 50%, and in future distributed base station architectures having an increasing demand for optical fibers, the transmission bandwidth utilization of the CPRI interface is very low.